1. Field of the Invention
The present invention relates to an image fiber in which a twisted portion is provided in order to rotate the orientation of a transmitted image, or to improve the quality of a transmitted image.
2. Background Art
Image fibers, to which a twisting process is applied, are known in the art. One of the purposes of applying such a twisting process is to rotate the orientation of a transmitted image at the object side of an image fiber with respect to that at the observer's side thereof. Another purpose is to improve the quality of a transmitted image such as contrast, etc., as disclosed, for example, in Japanese Unexamined Patent Application, First Publication No. 2000-185931. Contrast in a transmitted image is improved because undesired light, which leaks from a core and passes through a cladding, is reduced by applying an appropriate amount of twisting.
An image fiber twisting process includes removing a portion of a protective layer extending in the longitudinal direction of the image fiber to expose a body of the image fiber, softening the exposed portion by heating using a heat source such as an oxyhydrogen flame burner, twisting the image fiber about the center axis thereof, and cooling the image fiber for plastic fixation to obtain a twisted portion.
When the twisted portion is formed through the twisting process at an excessively large rate of twist, i.e., when the degree of twisting is excessive, deformation such as bending, elongation, etc., of cores that form pixels tend to become too great, and problems may be encountered in that defects such as dark spots are included in the transmitted image because light cannot pass through the cores. Another defect is that contrast in the transmitted image is reduced because light leaking from some cores affects adjacent cores.
In particular, the above-mentioned defects tend to be significant in the cores disposed near the periphery of the image fiber because deformation due to twisting is greater in the cores near the periphery than in the cores disposed around the center of the image fiber. The above-mentioned defects may occur where the degree of twisting is locally significant in the twisted portion.